Experimental and Computational Exploration of the NaF-ThF₄ Fuel System: Structure and Thermochemistry

The structural, thermochemical, and thermophysical properties of the NaF-ThF₄ fuel system were studied with experimental methods and molecular dynamics (MD) simulations. Equilibrium MD (EMD) simulations using the polarizable ion model were performed to calculate the density, molar volume, thermal expansion, mixing enthalpy, heat capacity, and distribution of [ThF] complexes in the (Na,Th)F melt over the full concentration range at various temperatures. The phase equilibria in the 10-50 mol % ThF₄ and 85-95 mol % ThF₄ regions of the NaF-ThF₄ phase diagram were measured using differential scanning calorimetry, as were the mixing enthalpies at 1266 K of (NaF/ThF₄) = (0.8:0.2), (0.7:0.3) mixtures. Furthermore, the β-Na₂ThF₆ and NaTh₂F₉ compounds were synthesized and subsequently analyzed with the use of X-ray diffraction. The heat capacities of both compounds were measured in the temperature ranges (2-271 K) and (2-294 K), respectively, by thermal relaxation calorimetry. Finally, a CALPHAD model coupling the structural and thermodynamic data was developed using both EMD and experimental data as input and a quasichemical formalism in the quadruplet approximation. Here, 7- and 8-coordinated Th⁴⁺ cations were introduced on the cationic sublattice alongside a 13-coordinated dimeric species to reproduce the chemical speciation, as calculated by EMD simulations and to provide a physical description of the melt.